Nuclear medical imaging is a useful technique that is applied in a variety of diagnostic contexts and medical specialties. In one type of nuclear medical imaging known as Single Photon Emission Computed Tomography (SPECT), the primary imaging task is to accurately determine and depict the spatial distribution of a radioactive isotope (radioisotope) used as a tracer (radiotracer) in the imaged object. A radiotracer may also be referred to as a radiopharmaceutical, as it is a pharmaceutical labeled with a radioactive isotope. For example, a patient may be injected with a radioactive pharmaceutical tracer (radiopharmaceutical or radiotracer) containing a radioisotope such as Technetium-99m (Tc-99m). Typically, the tracer travels to a target within the patient's body, and an attached radioactive atom emits gamma ray photons as it undergoes nuclear decay. A gamma camera (also known as an Anger camera or scintillation camera) has one or more detectors located near the patient for detecting emitted gamma rays that have traveled through a portion of the patient's body. The detector(s) are typically flat crystal plane(s) (e.g., 40 cm×50 cm crystals of NaI) that absorb counts of gamma ray photons. The absorbed gamma ray photons cause the crystal of the detector to scintillate, as an electron is dislodged from an atom (e.g., iodine atom) of the crystal and a pulse of light is produced in accordance with the Photoelectric effect and/or Compton effect. The light is detected, e.g., by an array of photomultiplier tubes (PMT) of the gamma camera. A collimator is typically mounted in front of the detector for limiting gamma ray detection to those photons emanating from the acceptance angle of collimator holes in a given field of view.
For SPECT imaging, gamma camera data are acquired from various view angles or projections and reconstructed in various planes in accordance with the principles of three-dimensional (3D) tomography. A gamma camera used for SPECT is also referred to as a SPECT camera. Gamma cameras for SPECT are described in more detail in Sayed, M., “Quality Control in Nuclear Medicine II. Planar and Single Photon Emission Computed Tomographic Gamma Camera Systems,” Turkish Journal of Nuclear Medicine 6(3): 185-189 (1997), the entire contents of which are hereby incorporated by reference herein.
Traditionally, the radiotracer distribution has been depicted in units of “proportional counts-per-second (prop-CPS),” i.e., units that are proportional to the radioactivity distribution, and the traditional image is only visually interpretable by the physician because it does not contain reliable absolute quantitative information (i.e., information with reliable units that have a physical basis with its associated measurement uncertainty). In other words, traditional SPECT images depict relative variations in activity concentrations to the trained human observer (e.g., using varying colors or shades of gray) but do not convey absolute quantitative information (i.e., the exact value at a particular voxel does not have a quantifiable physical meaning) Although it is possible to perform semi-quantitative assessment of traditional SPECT images by evaluating lesion-to-background ratios or by comparing the images to databases of normal patient scans, absolute quantification of tracer uptake and in turn disease diagnosis and monitoring has not been possible using traditional methods. An added uncertainty in quantifying the images lies in the fact that until now there has been no method to standardize the performance of SPECT cameras in the field. In other words, there has not been a common baseline across the field for SPECT image acquisition.